L5987 View Datasheet(PDF) - STMicroelectronics
Part Name
Description
MFG CO.
'L5987' PDF : 40 Pages View PDF
L5987
Application information
3. Calculate C4 by placing the zero at 50% of the output filter double pole frequency (fLC):
Equation 23
C4 = --------R-----14-------f--L---C--
4. Calculate C5 by placing the second pole at four times the system bandwidth (BW):
Equation 24
C5 = 2------------R-----4-------C----C4----4---4--------B----W--------–----1--
5. Set also the first pole at four times the system bandwidth and also the second zero at
the output filter double pole:
Equation 25
R3 = -4---------f----L--B----C-R-----W----1-------–-----1-
C3 = 2------------R-----3--1-----4-------B-----W----
The suggested maximum system bandwidth is equal to the switching frequency divided by
3.5 (FSW/3.5), anyway lower than 100 kHz if the FSW is set higher than 500 kHz.
For example: with VOUT = 3.3 V, VIN = 12 V, IO = 3 A, L = 10 H, COUT = 22 F, ESR < 1 m,
the type III compensation network is:
R1 = 4.99k R2 = 1.1k R3 = 220 R4 = 3.3k C3 = 3.3nF C4 = 10nF C5 = 180pF
DocID14972 Rev 4
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